Dominating stimulated Raman scattering of O-H symmetric and antisymmetric stretching vibrations in water molecule by pH-dependent hydrogen-bonding network.

Stimulated Raman Scattering (SRS) characteristic features of water molecules in acid and alkaline solution environments are investigated. SRS was switched from existence of one single peak (3364 cm-1) to coexistence of two peaks (3295 cm-1 and 3367 cm-1) and three peaks (3227 cm-1, 3319 cm-1, and 3429 cm-1) via pH adjustment. The single SRS peak attributes to O-H antisymmetric stretching vibration at 3364 cm-1 with pH = 7.0. SRS spectra exhibit the characteristic peak of the O-H symmetric stretching vibration locating from 3227 cm-1 to 3295 cm-1 when pH = 3.6, 4.0, 10.4, and 11.0, because the hydrogen and hydroxide ions lead to a stronger network structure of hydrogen bonding (HB) in water molecules. At the same time, the SRS threshold lowered to 7.50 mJ and 7.40 mJ at pH = 4.0 and 11.0. The optimized structures of water clusters were calculated by the density functional theory, and the simulated Raman spectra of (H2O)5, H3O+-(H2O)5, and OH--(H2O)5 structures match the SRS results at pH = 7.0, 4.0, and 11.0, respectively. The results reveal that acid or alkaline environment affects the intermolecular forces within the HB network, regulating symmetric and antisymmetric stretching vibration modes of O-H. This study provides a novel insight into the HB network and can be extended to other organic solvents.